“Wastewater” has always been recognized to have some value. In 1860, as waterborne sewer systems were becoming the norm, an alderman named Mechi told Farmer’s Magazine that “if the money value of our sewers could be shown to the British farmer in bright and glittering heaps of sovereigns, he would gasp at the enormous wealth, and make great efforts to obtain the treasure.” Mechi was talking about the fertilizing nutrients in human “waste,” which he thought were needlessly ruined by mixing excrement with water, but he might also have been talking about its wasted energy potential.
Sludge, the solids that remain after sewage has been cleaned into effluent, has a high B.T.U. content (a measurement of fuel’s energy); it burns efficiently and well. Other aspects of wastewater treatment can also reap energy: anaerobic digestion (whereby bacteria munch on the organic contents) produces methane, which with turbines can become combined heat or power. Microbial fuel cells can use bacteria to get electricity from sewage, while gasification, a high-temperature process, can reap fuel-ready gas from sludge.
When it comes to harnessing energy from wastewater treatment, it sounds as if we are spoiled for choice. Then you look at the numbers. Of the 16,000 wastewater treatment plants in the United States, about 1,000 process enough gallons (five million daily) to be able to generate cost-effective energy using anaerobic digestion. Yet only 544 use anaerobic digestion, and only 106 of those do anything more with the gas produced than to flare it.
If those 544 treatment plants generated energy from their sewage, the E.P.A. concluded in a 2007 report, they could provide 340 megawatts of electricity (enough to power 340,000 homes), and offset 2.3 million tons of carbon dioxide that would be produced through traditional electricity generation. In the effort to reduce greenhouse gases, the E.P.A. said, this would be equivalent to planting 640,000 acres of forest or taking some 430,000 cars off the road.
Gasification, like anaerobic digestion, is an age-old process. It used to supply gas lamps in some American towns, too, before piped gas became the norm. The process — a thermal conversion at high temperatures — could probably be done in a garbage can. But the utilities haven’t been eager to push the technology. The sewage treatment process — essentially, filter, settle, digest — hasn’t changed much since the early 1900s, because it works. And drying out sludge enough to make it burnable takes money and energy. Pilot projects may take several years to pay for themselves, which can clash with short-term budget cycles.
Other factors may force the industry’s hand. It takes considerable energy to clean sewage, and energy costs have risen along with global temperatures. Now isolated pioneers are showing how investing in “waste” can pay off: London’s Thames Water utility now generates 14 percent of the power it needs from burning sludge or methane, saving $23 million a year in electricity bills.
Also, it’s green to burn the brown stuff. Resource recovery from wastewater counts as renewable energy, which makes sense: we’re hardly likely to stop providing the raw material anytime soon. So why continue to flush away a resource whose value, even under the dim light of a sewer gas lamp, should be blindingly obvious?